JSON Schema tools and doc generation for HTTP APIs
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JSON Schema tooling: scaffold, verify, and generate documentation from JSON Schema documents.


JSON Schema provides a great way to describe an API. prmd provides tools for bootstrapping a description like this, verifying its completeness, and generating documentation from the specification.

To learn more about JSON Schema in general, start with this excellent guide and supplement with the specification. The JSON Schema usage conventions expected by prmd specifically are described in /docs/schemata.md.


Install the command-line tool with:

$ gem install prmd

If you're using prmd within a Ruby project, you may want to add it to the application's Gemfile:

gem 'prmd'
$ bundle install


Prmd provides four main commands:

  • init: Scaffold resource schemata
  • combine: Combine schemata and metadata into single schema
  • verify: Verify a schema
  • doc: Generate documentation from a schema
  • render: Render views from schema

Here's an example of using these commands in a typical workflow:

# Fill out the resource schemata
$ mkdir -p schemata
$ prmd init app  > schemata/app.json
$ prmd init user > schemata/user.json
$ vim schemata/{app,user}.json   # edit scaffolded files

# Provide top-level metadata
$ cat <<EOF > meta.json
 "description": "Hello world prmd API",
 "id": "hello-prmd",
 "links": [{
   "href": "https://api.hello.com",
   "rel": "self"
 "title": "Hello Prmd"

# Combine into a single schema
$ prmd combine --meta meta.json schemata/ > schema.json

# Check it’s all good
$ prmd verify schema.json

# Build docs
$ prmd doc schema.json > schema.md

Using YAML instead of JSON as a resource and meta format

init and combine supports YAML format:

# Generate resources in YAML format
$ prmd init --yaml app  > schemata/app.yml
$ prmd init --yaml user > schemata/user.yml

# Combine into a single schema
$ prmd combine --meta meta.json schemata/ > schema.json

combine can detect both *.yml and *.json and use them side by side. For example, if one have a lot of legacy JSON resources and wants to create new resources in YAML format - combine will be able to handle it properly.

Render from schema

$ prmd render --template schemata.erb schema.json > schema.md

Typically you'll want to prepend header and overview information to your API documentation. You can do this with the --prepend flag:

$ prmd doc --prepend overview.md schema.json > schema.md

You can also chain commands together as needed, e.g.:

$ prmd combine --meta meta.json schemata/ | prmd verify | prmd doc --prepend overview.md > schema.md

See prmd <command> --help for additional usage details.

Documentation rendering settings

Out of the box you can supply a settings file (in either JSON or YAML) that will tweak the layout of your documentation.

$ prmd doc --settings config.json schema.json > schema.md

Available options (and their defaults)

  "doc": {
    "url_style": "default", // can also be "json"
    "disable_title_and_description": false, // remove the title and the description, useful when using your own custom header
    "toc": false // insert the table of content for json scheme documentation to the top of the file. (default disable)

Use as rake task

In addition, prmd can be used via rake tasks

# Rakefile
require 'prmd/rake_tasks/combine'
require 'prmd/rake_tasks/verify'
require 'prmd/rake_tasks/doc'

namespace :schema do
  Prmd::RakeTasks::Combine.new do |t|
    t.options[:meta] = 'schema/meta.json'    # use meta.yml if you prefer YAML format
    t.paths << 'schema/schemata/api'
    t.output_file = 'schema/api.json'

  Prmd::RakeTasks::Verify.new do |t|
    t.files << 'schema/api.json'

  Prmd::RakeTasks::Doc.new do |t|
    t.files = { 'schema/api.json' => 'schema/api.md' }

task default: ['schema:combine', 'schema:verify', 'schema:doc']

File Layout

We suggest the following file layout for JSON schema related files:

/docs (top-level directory for project documentation)
  /schema (API schema documentation)
      /{resource.[json,yml]} (individual resource schema)
    /meta.[json,yml] (overall API metadata)
    /overview.md (preamble for generated API docs)
    /schema.json (complete generated JSON schema file)
    /schema.md (complete generated API documentation file)

where [json,yml] means that it could be either json or yml.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request